docs: def _get_bandfile(self, **options):
          """Get the VIIRS rsr filename"""
  
          # Need to understand why there are A&B files for band M16. FIXME!
          # Anyway, the absolute response differences are small, below 0.05
  
          # LOG.debug("paths = %s", str(self.bandfilenames))
  
          path = self.bandfilenames[self.bandname]
          if not os.path.exists(path):
              raise IOError("Couldn't find an existing file for this band ({band}): {path}".format(
                  band=self.bandname, path=path))
  
          self.filename = path
          return